Recited in Patent Literature 1 as one example of systems which measure a propagation time of ultrasonic to determine a position of a moving body is an electronic pen system. According to the position determination method which uses a propagation time of ultrasonic as recited in Patent Literature 1, an electronic pen as a transmitter simultaneously transmits an ultrasonic signal and a trigger signal in a fixed cycle to measure a time from a time point of reception of the trigger signal by a receiver and software until a time point of reception of the ultrasonic signal transmitted from the electronic pen as a propagation time of ultrasonic and determine a position of the electronic pen by using the propagation time.
The ultrasonic signal transmitted from the electronic pen, which is a signal whose waveform has such a burst shape as shown in FIG. 17, is transmitted with the same waveform in each transmission cycle. The receiver first receives a trigger signal and then receives an ultrasonic signal arriving with a delay according to a propagation distance as shown in FIG. 18.
Further recited, as another example of position determination by measuring a propagation time of ultrasonic transmitted from a plurality of emission sources, is an ultrasonic type coordinate input device which enables use of a plurality of pens at the same time in Patent Literature 2. In the position determination method using a propagation time of ultrasonic recited in Patent Literature 2, a fixed body (receiver) transmits an electromagnetic wave signal such as an infrared signal including ID information and a moving body (pen) side transmits ultrasonic only when receiving an electromagnetic wave signal relevant to its own ID. The interval of transmission of an electromagnetic wave signal including ID information is set to be longer than a time of moving of ultrasonic within the maximum range of coordinate input.
As shown in FIG. 19, with a sampling cycle T for measuring a coordinate, a time assigned to one pen will be T/n which is determined by a relationship with a drawing range. In a case where T is 10 ms and the drawing range is A4 size, a propagation distance of ultrasonic will be 350 mm at the maximum and a propagation time will be on the order of 1 ms, so that when T/n is set to be 2 ms taking into consideration a margin to prevent the time from falling in a time frame of other ID, five pens can be used simultaneously at the maximum.
On the other hand, assuming the use of an electronic pen on a projection screen of a projector, a screen size on the order of 80 inches will be required as a drawing range in practice, so that a propagation distance of ultrasonic transmitted from the electronic pen to reach a receiver will be about 2 m at the maximum and a propagation time will be on the order of 7 ms.
One example of methods of calculating a propagation time of a sound wave is recited in Patent Literature 3. The sound wave propagation time calculation method recited in Patent Literature 3 is a method in which with an M sequence phase-modulated wave used as a transmission wave, a receiver receives an M sequence phase-modulated wave transmitted by a transmitter and a matched filter obtains correlation between a reception signal and a transmission signal to detect a peak of an output of the matched filer and calculate a propagation time of a sound wave by a peak time.    Patent Literature 1: U.S. Pat. No. 6,118,205    Patent Literature 2: Japanese Patent Laying-Open No. 2004-199560    Patent Literature 3: Japanese Patent No. 3876370    Non-Patent Literature 1: Hiroshi Kashiwagi, “M-sequence and Its Applications”, Mar. 25, 1996, Shokodo
In a case of measuring a propagation time of ultrasonic from moving bodies as a plurality of determination targets, when an ultrasonic detection range is large, a propagation time of ultrasonic from the most distant point will be longer to require the above-described T/n to be set to be longer. The detection interval T, however, cannot be set to be long unnecessarily when, for example, preventing collision of bodies or reproducing handwriting by an electronic pen. When the most distant point locates as far as 2 m among as in a case where an electronic pen is used on a projection screen of a projector, the propagation time will be on the order of 7 ms, so that only one electronic pen can be used taking a margin into consideration.
Thus, according to the method recited in Patent Literature 1, when a propagation distance of ultrasonic is long, it is so difficult to detect positions of a plurality of moving bodies (electronic pens) precisely and stably that the pens cannot be used.
According to the method recited in Patent Literature 2, because as an ultrasonic transmission signal, the same signal waveform is assumed for a plurality of pens, detection of an arrival point of ultrasonic from different pens might be affected. A receiver, for example, receives a reflected wave of ultrasonic transmitted first which is propagated through a long distance prior to a direct wave transmitted from a different pen having a next ID and erroneously recognizes the first received reflected wave as a direct wave to erroneously measure an arrival time, so that it is impossible to calculate an accurate position of an electronic pen.
Furthermore, since a wave of a burst shape is assumed as an ultrasonic transmission signal, a configuration of a composite wave will change according to overlapping between a direct wave and a reflected wave, which makes it difficult to detect an arrival point of a direct wave arriving first and impossible to determine an accurate position of an electronic pen as a moving body.
A method of calculating a propagation time of a sound wave by using an M sequence phase-modulated wave as a transmission wave such as the method recited in Patent Literature 3 is a method of measuring a propagation time of a sound wave on a different propagation path on which a transmitter/receiver is disposed in advance and measuring a flow velocity of fluid flowing through a target object on which a transmitter/receiver is disposed, which method, when a plurality of transmitters (moving bodies) transmit ultrasonic simultaneously, is not allowed to identify the plurality of transmitters and measure a propagation time of ultrasonic from each transmitter, that is, is not allowed to determine positions of a plurality of moving bodies precisely and stably.